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It's Autumn 2015, and I just saw a tree that had mainly dead brown leaves but also some new small green leaves growing. Why didn't the tree just keep its old leaves? Does it think we are in spring already?

EDIT: To be clear, I had always thought that in autumn that all the leaves of a tree would fall off together, as a preparation for winter. It seems silly that the tree would then immediately grow some new leaves when it just decided to let all those previous leaves die. Why not just use that same energy to maintain those leaves?

I've taken some photos of the leaves and the trunk for help with identification.

enter image description here

enter image description here

More images of the tree can be seen at this Imgur link

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    $\begingroup$ Welcome to Bio. Without a picture or species identification any answer is mere guessing. $\endgroup$ – AliceD Nov 18 '15 at 23:30
  • $\begingroup$ I don't think it would be localized to just California but a picture would definetly help. There could be so many reasons why the tree hasn't dropped it's dead foliage . $\endgroup$ – Technetium Nov 19 '15 at 16:44
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    $\begingroup$ I've edited the original post with a link to some photos! Tell me if you need more. $\endgroup$ – personjerry Nov 19 '15 at 23:48
  • $\begingroup$ What is your question? Are you asking why there are dead leaves and green leaves, are you asking why trees don't have their leaves last forever, or are you asking why the dead leaves didn't fall off? $\endgroup$ – iayork Nov 20 '15 at 12:59
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    $\begingroup$ I'm wondering why the tree seems to be growing new green leaves while it has many dying leaves. Why didn't it just use that energy to maintain the existing leaves? $\endgroup$ – personjerry Nov 20 '15 at 13:01
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Whether trees are deciduous or evergreen, their leaves don't last forever; they need to be replaced at intervals. The process of photosynthesis depletes the chlorophyll in the leaves, so that needs to be replaced and this is a significant metabolic cost to the tree. Therefore, whenever a tree is experiencing metabolic stress, it will slow down the chlorophyll replacement, and let the leaves die to save energy. It's metabolically cheaper to simply let the leaves die -- after removing the most precious nutrients -- and then regrow new leaves as needed, than to keep repairing the leaves that aren't needed because the tree is resting for other reasons.

In temperate areas, where there are well-defined seasons, this typically happens in the fall, and deciduous trees let their leaves die all at once in the fall. In tropical or subtropical regions, where trees could photosynthesize all year, leaves may drop all year round (as with evergreen trees, like conifers, in temperate regions). Often, though, their leaves will reflect metabolic stresses on the tree, such as drought.

(The Wikipedia article on "Deciduous" makes the point that this can be very localized, related to very local environmental differences that could only affect a single tree.)

Much of California is a subtropical region, and it has been experiencing severe drought for some time, so it wouldn't be surprising for a tree to have reached a point where its leaves have all had their chlorophyll removed. On the other hand, as the El Nino event of 2015 increases, rainfall in California is returning, so it would also not be surprising for a tree to have recovered from drought enough to start making new leaves.

If your question is why the old leaves didn't fall off before the new ones started to regrow, it is probably simply a matter of timing. The tree doesn't really "care" if the leaves fall or not, so long as they're not using up its resources, so there's no reason to force them off. The new water supply may have arrived fairly shortly after the degree of drought that caused the tree to shut down the leaves.

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  • $\begingroup$ Ah, I assumed that all the leaves were browning and falling off together because of the season. In this scenario, it seemed silly to start growing leaves again after letting them all die. If I'm understanding you correctly, in California this may be a continuous leaf dying/growing cycle throughout the year? And the recent rain (indeed, it rained for the first time in a few months last week) allowed the growth of some fresh new leaves? $\endgroup$ – personjerry Nov 20 '15 at 13:23
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    $\begingroup$ Exactly right. In fact, drought is functionally why leaves drop even in temperate zones -- in winter, all the water is locked up in the form of snow and ice that plants can't take advantage of, so they drop their leaves in preparation for this predictable period of drought. $\endgroup$ – iayork Nov 20 '15 at 13:25
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Depending on the part of California, most deciduous trees will go through the customary winter-dormancy cycle as is seen throughout the rest of the planet, albeit often majorly abbreviated. Even as far south as Needles many "eastern hardwoods" that had been planted by landowners will dutifully drop their leaves as winter approaches, renewing their greenery he following spring.

The problem you mentioned is not restricted to California: Deciduous trees and shrubs facing severe and/or prolonged drought conditions will "go dormant" earlier than would be normal. This is a simple "survival strategy," in which the plant reduces the potential for long-term injury or harm by---in effect---hibernating. Nature has built this survival strategy into a wide range of life-forms; periods of crisis-dormancy allows the subjects to "weather" adverse conditions and, "hopefully," revive when the previously-experienced problems had passed.

I've noted deciduous plants going through leaf-drop as early as July, in New York State. Recently-planted deciduous trees and shrubs do not have the extensive root-systems of established plants, and will quite often go through early leaf-drop because they will feel the effects of a drought far more keenly than established plants. That is not to suggest that your trees or shrubs were recent plantings: It is merely a statement of fact.

A curious thing about early-dormancy is that plants will "rebound" and set new leaves---essentially, sending forth some of the leaves that had been grown for next year's spring and summer cycle.

This will come about when there has been a very recent period of increased rainfall, coupled with cooler ambient temperatures.

If you will check the recent weather-tables for California, you will note this is exactly what happened: A long, hot, period of zero precipitation and arid humidity was broken by an interval of cooler and more-moist weather, going into late-summer and fall.

The California hardwoods had "rebounded," setting a reduced number of new leaves and, essentially, tried to make up for the brutal conditions of the previous spring and summer by getting a few more weeks of photosynthesis and growth before Autumn arrived without doubt, and the cycles of shorter days and reduced temperatures obliged the plants to enter "true" dormancy.

My 12-15 hours of "Dendrology" (the study of trees) were taken nearly five decades ago, although I'd striven to maintain a B+, even in "Science electives." Besides, I'd really enjoyed "woodsy" courses as it allowed me another valid excuse to "prowl the pines & ramble the brambles!"

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